1. Field of the Invention
This invention relates to a metal gasket for sealing a clearance between the opposed surfaces of parts of an engine, especially a multicylinder engine.
2. Description of the Invention
A metal gasket has been used to seal a clearance between the opposed surfaces of structural parts of an engine, such as a cylinder head and a cylinder block. A metal gasket has beads at the portions thereof which are in the vicinity of its through holes made correspondingly to combustion chambers, water passages and oil passages. When the metal gasket is inserted between a cylinder head and a cylinder block and tightened with bolts and thereby fixed, the beads form annular, elastic contact portions with respect to the opposed fixing surfaces and work so as to seal the clearance between the same opposed fixing surfaces.
A recent engine requires a higher output and smaller weight. There is a tendency for a cylinder head to be manufactured out of an aluminum material of a low specific gravity instead of a conventional material of a high specific gravity, such as steel and a cast iron for the purpose of meeting the demand. An aluminum material has small weight but does not a high rigidity, so that the relative displacement of a cylinder head with respect to a cylinder block comprising steel or a cast iron tends to increase during an operation of an engine. The tightening bolts for tightening the opposed fixing surfaces of these structural members of different materials via a single-plate metal gasket are set in a scattered manner wholly in an outer circumferential portion of the metal gasket or comparatively overwhelmingly in an outer circumferential portion thereof. Namely, the tightening bolts are not always distributed uniformly around the bore holes, so that the opposed fixing surfaces are liable to become irregular. Consequently, a high-temperature high-pressure combustion gas enters the portions, which are greatly distorted, of the opposed fixing surfaces which are among the bore holes to cause the beads of the metal gasket interposed between the opposed fixing surfaces to be corroded and polluted, and the sealing effect of the metal gasket to decrease.
When a cylinder head gasket is used, the distance between a cylinder head and a cylinder block increases and decreases repeatedly during a combustion cycle of an engine, and repeated stress (mechanical stress and thermal stress) is exerted on the metal gasket as well. The load fluctuation stress against the metal gasket occurs at the highest level in the portion thereof which corresponds to the portions of the lowest rigidity of the cylinder block and cylinder head. As a result, permanent set in fatigue and cracks occur in the beads formed on the metal gasket, to cause the sealing performance of the metal gasket to be deteriorated.
The applicant of the present invention developed the metal gasket shown in FIGS. 8, 9 and 10, and filed previously (Japanese Patent Laid-Open No. 39868/1993). FIG. 8 shows a metal gasket having a structure in which a pair of elastic metal plates 21, 28 are laminated, the boundary portions of adjacent bore holes 22A, 22B; 29A, 29B formed in these elastic metal plates 21, 28 being shown in section along the center line connecting the centers of the same bore holes 22A, 22B; 29A, 29B together. Around the bore holes 22A, 22B of the elastic metal plate 21, a bead 24 projecting toward the elastic metal plate 28 is formed annularly, and, around the bore holes 29A, 29B of the elastic metal plate 28, a bead 30 projecting toward the elastic metal plate 21 is formed annularly. The bead 24 of the elastic metal plate 21 and that 30 of the elastic metal plate 28 are provided so as to contact each other. The beads 24, 30 may be formed so as to be aligned with each other as shown in the drawing or staggered from each other between bore holes 22A and 22B and between bore holes 29A and 29B. Around the bore holes 22A, 22B of the elastic metal plate 21, folded portions 25A, 25B folded back toward the elastic metal plate 28 are formed.
In the above-described metal gasket, a double seal portion is secured in the bead and folded portions around the bore holes, so that, even when irregularity occurs on the opposed fixing surfaces of two structural parts during a gasket tightening operation, the bead and folded portions are deformed correspondingly to the distorsion of these fixing surfaces to offset the irregularity of the same surfaces. Since the forming of such folded portions as mentioned above is restricted by the hardness, extensibility, strength and thickness of the elastic metal plate, it is necessary that a plate material having comparative-low hardness and strength, a small thickness and a high extensibility be used. When a metal gasket formed out of such a material is used for a long period of time, permanent set in fatigue and cracks become liable to occur in the bead. Moreover, the folded portions formed on a metal gasket make the thickness of the gasket increase around the bore holes. Therefore, when the metal gasket is held between the opposed fixing surfaces and tightened, the surface pressure is more easily concentrated on the bore hole-surrounding portions than on the other portions of the elastic metal plates. When the engine parts are formed out of aluminum, impressions become liable to occur in the opposed fixing surfaces.
The metal gasket shown in FIG. 9 differs from that shown in FIG. 8 in that the thickness of an elastic metal plate 31 is set equal to that of an elastic metal plate 38.
The metal gasket shown in FIG. 10 differs from that shown in FIG. 8 in that soft metal plates 27A, 27B are embraced in folded portions 25A, 25B of an elastic metal plate 21 so as to regulate the thickness of the portions of the metal gasket which are around the bore holes 22A, 22B. When the thickness of the portions of the metal gasket which are around the bore holes 22A, 22B is increased, the sealing performance of the metal gasket held between the opposed fixing surfaces and tightened can be improved with respect to the leakage of a gas from the portions of the same surfaces which are around the bore holes 22A, 29A; 22B, 29B.
Japanese Patent Laid-Open No. 243531/1995 discloses the scalability of the circumfenential edge portions of the bore holes of a metal gasket. In this metal gasket, opening edge-applying metal plates for surrounding at their regions of a suitable width the circumferential edge portions of cylinder-aligned openings equivalent to bore holes, and a metal plate joined to the circumferences of the opening edge-applying metal plates, and having a thickness smaller than that of the opening edge-applying metal plates constitutes an auxiliary plate. This metal gasket is formed by laminating this auxiliary plate between two base plates having beads projecting toward the cylinder-aligned openings of the other base plates, in such a manner that the beads contact the opening edge-applying metal plates.
In general, due to the reduction of the weight of a recent engine, the distance between adjacent bore holes tends to decrease, so that the regions between adjacent bore holes are exposed to a high-temperature combustion gas to cause the gas to become liable to leak. Therefore, in order to secure the sealing performance among the bore holes, expensive elastic metal plates are used for the production of a metal gasket. In the metal gasket disclosed in this publication, an expensive elastic metal plate is used by extending itself over the whole opposed fixing surfaces so as to seal the holes which are other than the bore holes, and which do not require high-degree sealing performance of the metal gasket. Therefore, the metal gas-manufacturing cost becomes high.